We have examined the reactivity of D-2 on CO-precovered Pt(5 3 3) employing supersonic molecular beam techniques and compare our results to D-2 dissociation on the bare surface. At very low kinetic energy, reaction probability is reduced linearly with increasing CO-precoverage at step edges. Diffusion of molecularly bound hydrogen along the step edge can therefore be excluded. In addition, CO adsorption at steps increases the barrier to direct dissociation at terrace sites by similar to 14 kJ/mol. A comparison to results from a previous study [A.T. Gee, B.E. Hayden, C. Mormiche,T.S. Nunney, Surface Science 512, 165 (2002)], shows different poisoning behaviors by CO and O that may be related to their ability to react with hydrogen. (C) 2010 Elsevier B.V. All rights reserved.